Surface forces and charging properties of toners are modified by application of fine particulate surface treatments. The most common surface treatments are surface modified fumed silica powders, but fine particles of titania, alumina, zinc oxide, tin oxide, cerium oxide, and polymer beads can also be used. Surface treatment may serve other functions such as providing cleaning aids to ancillary processing in an electrophotographic process.
Surface treatments are used to reduce surface forces for improved powder flow and transfer efficiency. High transfer efficiencies are not only desirable for yield but also for improved image quality by minimizing transfer variation sources of density non-uniformities. Variations in transfer can be thought of as acting upon the untransferred or residual toner. Obviously, non-uniformities due to poor transfer are reduced by improved transfer. Additionally, high transfer variations from such sources as fuser oil and vibrations that induce shear transfer have less residual to act upon to generate density non-uniformities.
Many aspects of surface treatment are spelled out in U.S. Pat. No. 7,601,473, the disclosure of which is incorporated herein in its entirety by reference. The surface treatment states of free, tacked, embedded, and engulfed are described in this patent and the impact of each state on the performance of the toner in an electrophotographic process is given. The use of two or more types of surface treatment particles with different degrees of tacking is described to obtain a balance between surface forces and charging properties of toner in an electrophotographic device.
Surface force modification by surface treatment occurs due to separation of the toner surface from other surfaces. This separation reduces the adhesive and cohesive forces on the toner and improves transfer of toner from the photoconductor to intermediate and final receivers. As the surface treatment is embedded, these forces increase, reducing powder flow and transfer performance of the toner.
When added to the toner with a relatively low energy and low temperature mixing process, surface treatment particles are weakly adhered to toner surfaces and are thus free to transfer to other surfaces, such as magnetic carrier particles in a two-component toner-carrier mixture, or to a photoreceptor or intermediate transfer member surface. In this state we define surface treatment particles as being free. When added to toner using a higher temperature and higher energy mixing process, surface treatment particles can become “tacked” to the toner surface and thus transfer less readily to other surfaces. In the tacked state surface treatment particles however still function to achieve separation of the toner surface from other surfaces such as the photoconductor or other toner particles, and thus are effective in improving performance in aspects like transfer efficiency and bulk toner powder flow. The use of even higher energy and higher temperature mixing processes will result in the surface treatment particles become physically embedded in the toner particles, a condition which results in the loss of separation of surfaces and a resulting loss of performance in properties like transfer efficiency and bulk powder flow. It is observed that the use of a two component toner-carrier developer mixture in a toning station results in embedment of surface treatment particles due to the energy of collisions between particles in mixing zones, transporting augers, and other energy imparting sections of the toning equipment. The result is the loss of performance due to the loss of the surface treatment separating function just described. Finally, by the application of even more energy surface treatment particles can become completely engulfed within the toner particles, as seen for example in an electron microscope. In this state the desirable properties of the surface treatment particles are lost completely. The progression of free to tacked to embedded to engulfed surface treatment states is a continuum; as well a given toner particle can have surface treatment particles present in all of these states simultaneously. In the present invention we define a degree of tacked state versus free state surface treatment by a quantitative measurement of the transfer of surface treatment agents from a toner surface to a clean carrier particle surface. It is an object of the present invention to provide surface treated toner with a high degree of tacked surface treatment such that the beneficial effects due to separation are realized, while minimizing the deleterious effects of large quantities of free surface treatment such as filming on the photoreceptor and transfer intermediate member surfaces.
The surface treatment particles are often treated with chemical modifiers to reduce their surface energy and improve their performance as powder flow aids. The impact of these modifiers on tacking and embedment are described in U.S. Pat. No. 7,601,473. The surface energy of powders may be characterized by a mid point and a range of surface energy between no wetting and complete wetting by mixtures of water and methanol. Table 1 below gives values for various types of commercially available silica useful as surface treatments.
Also described in U.S. Pat. No. 7,601,473 are various toner and surface treatment formulations, and processing equipment and conditions needed to obtain the desired tacking state of the surface treatment. Tacking the surface treatment in place once uniformly dispersed on the toner surface under controlled conditions with low shear allows the use of lower surface treatment concentrations. Tacking will also prevent transfer of the surface treatment to other surfaces. However, the tacking initiates the embedment process and reduces the number of impacts a toner particle may sustain before the surface treatment becomes ineffective at maintaining the desired separation from other surfaces.
The collision energy required to tack the surface treatment may be reduced by increasing the temperature of the fluidized bed. At elevated temperature, less kinetic energy from collisions is required to generate sufficient heat at the contact point with the surface treatment to exceed the toner resin Tg. U.S. Pat. No. 7,601,473 teaches processing toner in a fluidized bed of elevated temperature ranging from 15° C. less than the Tg to the Tg of the toner with two or more surface treatment components to obtain the desired combination of tacked silica for improved powder flow and transfer performance with improved tribocharging.
The average degree of embedment varies with the residence time of the toner in a process. The longer the toner is in a process, the more collisions it undergoes and the greater the embedment. The residence time varies in a toning station is inversely proportional to the image content of the documents being printed with that toner. As a result, the surface treatment will undergo embedment and engulfment at long residence times. Processes that aerate the toner are most often used to surface treat toners with small particles. These devices rely upon particle-to-particle and particle-to-mixing member collisions. The kinetic energy of such collisions is proportional to the mass of the toner particle, hence to the cube of the toner size.
Larger surface treatment particles may sustain many more impacts before embedment reduces their effectiveness. As the size of surface treatment particles increase, the area of contact increases and the energy of collisions must be increased to bring the localized temperature above the Tg required for increasing the degree of embedment. Consequently, larger surface treatment particles are more difficult to tack to the toner. There is greater leverage for impacts to dislodge a larger surface treatment particle and a larger contact area is needed between the surface treatment and toner for the Van der Waal's forces to provide tacking. A greater amount of toner material must be displaced by the surface treatment to provide the required contact area. The amount of non-tacked surface treatment is driven by the width of the surface treatment particle size distribution because the large particles within the surface treatment require higher temperatures to tack and embed.
The total surface treatment that may be applied is limited by the ability to tack the silica. As the total projected area of the surface treatment approaches the outer surface area of the resin core particles of the toner, there is less exposed toner on which to tack the surface treatment. A secondary effect is that the collision forces during mixing are distributed over more contact points and less toner material is displaced by a given surface treatment particle during the collision resulting in lower contact area. This distribution of collision forces is greater for surface treatments having a narrow size distribution. For broad or bimodal surface treatment size distribution, the collision forces are concentrated on the large surface treatment particles where more energy is needed to affect tacking. Larger surface treatment particles are also known to protect smaller surface treatment particles from embedment due to the separation effect.
For toners with surface treatment in the free state, the surface treatment may be transferred to the carrier in two component developers modifying the developer flow and toner concentration sensor performance. The free surface treatment may also transfer to other soft surfaces and accumulate in a film. When the soft surface is a photoconductor, this film may lead to variations in imaging performance.
U.S. Pat. No. 5,066,558 teaches the use of a three-step process first to disperse a silica powder on a resinous core toner particle in a lower energy device, second to embed the silica in a second higher energy device such that there are little or no visible silica particles on the surface by SEM, and third to disperse additional silica powder in a device similar energy to that used in the first step. The method pertains to single component developers of 100 wt % toners and as such does not address issues of toner concentration control.
U.S. Pat. No. 6,087,057 teaches the use of two treated silica powders where the first silica powder is treated with an alkyl silane and an amino alkyl silane to give a negative charge and the second silica powder is treated with an organopolysiloxane that charges positive relative to the first and an third metal oxide to adjust charge. These formulas are selected solely for tribocharge stability upon admix, changes in relative humidity (RH), etc.
U.S. Pat. No. 6,582,866 teaches toner and processes for making toner where the toner comprises surface additive particles adhered to combined colorant and resin particles in a quantity greater than three percent of the combined weight of resin and colorant in the toner, where the surface treated toner is obtained by an impaction process employing a high intensity blending tool.